Hydrogen (H2) is a colorless, odorless gas that is the most abundant resource
in the universe. On earth, it is mostly found as water. It is not usually found
in its pure form because it combines easily with other elements. It can, however,
be reproduced from renewable resources.

Hydrogen has one of the highest energy density values per mass. Its energy
density is between 120 and 142 MJ/kg. This means that for every 1 kg of mass
of hydrogen, it has an energy value of 120-142 MJ. It is highly flammable, needing
only a small amount of energy to ignite and burn. Hydrogen burns cleanly. When
it is burned with oxygen, the only by products are heat and water.

Today, hydrogen is mainly used as a feedstock, intermediate chemical, or specialty
chemical. The US hydrogen industry produces nine million tons of hydrogen per
year for use in chemical production, petroleum refining and electrical applications.

Currently, many people advocate a hydrogen economy, which will use hydrogen
as an energy carrier or fuel. It can be safely stored and transported. Using
hydrogen fuel would also improve air quality. However, since there are no naturally
occurring sources of hydrogen, it must be made. The production of hydrogen is
more costly than using fossil fuels. For now, NASA is the primary user of hydrogen
as an energy carrier.